Munna..you can never really compare two 308ds with 1 15" type R.
if the box tuning is the same for both 308s and type R...type R will always be louder, and much much cleaner.
even with that fusion amp, you had bass boost full and gain full...
i have always noticed about 308s that it may go loud, but it loses the definition of bass notes. it always makes this bbrrrr brrrbr when you increase the volume. and the kick is lost in the brrrr brrr...true it can go loud. but not louder than the type R. my bass boost was at 25% and gain at 60%. and thats my normal everyday setting.and also at G2G that was my setting 
sub placement matters alot in my car..
not trying to say my setup is best. just making a general observation that 15" type R is louder than two 308s - and musical at the same time. kevin,mtx,quereshi all say this. but i had never made a true test of this so couldnt comment on it. thats why i had come with SPL in mind and wanted to compare the two subs 